Whilst testing an app on the Jelly Bean emulator, I couldn’t get the emulator to mount the SD card. Calls to Environment.getExternalStorageState() would always return Environment.MEDIA_REMOVED.
After some searching, I realised the found was being configured with hw.sdCard=no during creation, despite specifying an SD card in the UI.
To fix the issue, open up the config.ini for the affected emulator, and change the value to hw.sdCard=yes, e.g:
# ~/.android/avd/Nexus_5X_API_16.avd/config.ini
# ...
hw.sdCard=yes
After restart your emulator, it should now successfully mount the SD Card.